UNITY 3D _ Instantiate GameObject at Random time is not working

Whats up guys , I'm new to Unity and this is my first game so forgive my stupid question , I want the enemy to spawn at random time between 1-5 seconds but when I applied the script inside the game , the game is spammed with enemies and it continuously nonstop spawning enemies !

I put the script and a a screenshot to understand whats the problem exactly , Thanks for your time :)

 using UnityEngine; using System.Collections;

 public class Game_Control : MonoBehaviour {

 private bool Spawn1=true;

 public GameObject Spearman;

 private float STimer;

 void Start () {

 STimer =  Random.Range(1f,5f);
           }

   void Update () {

  if (Spawn1 = true) {
     Instantiate (Spearman, transform.position, transform.rotation);
     StartCoroutine (Timer ());} 

   }

 IEnumerator Timer() {

 Spawn1 = false;
 yield return new WaitForSeconds (STimer);
 Spawn1=true; }
 }